In recent years thiourea has attracted considerable attention in hydrometallurgy of the precious metals gold and silver. One of the reasons for the given attention is the kinetic superiority of thiourea leaching compared to cyanide leaching. Another reason is thiourea's substantially less toxic nature than cyanide. Thus, thiourea may very well be used as a substitute for cyanide in many hydrometallurgical applications.
The hydrometallurgy techniques using thiourea share similarities to the already well known cyanidation techniques. In other words, thiourea can be used in similar types of unit operations that are used with cyanide. However, a significant difference between thiourea and cyanide leaching techniques is the pH of the leach solutions that are used. Cyanide must be used in basic solutions to prevent evolution of toxic gaseous hydrogen cyanide. In contrast, thiourea is used in acidified solutions.
The main unit operations in a thiourea leaching method are (1) thiourea leaching (by agitation, dump, or heap leaching), (2) recovery of dissolved gold and silver on activated carbon, (3) carbon elution, and (4) recovery of gold and silver from the elution solutions.
Adsorption of the precious metals on activated carbon can be done either in pulp (CIP), during leaching (CIL), or from leach solution obtained after solid liquid separation. Sometimes the adsorption of gold and silver onto carbon from the leach solution can be bypassed, and these metals can be recovered either directly (e.g. by electrowinning or chemical precipitation), or after preconcentration (e.g. by solvent extraction or ion exchange). Preferably, however, gold and silver is recovered using activated carbon.
After adsorption of gold and/or silver on activated carbon, the precious metals must be desorbed from the activated carbon. Desorption of carbon, also called elution, must be performed under different conditions from the conditions during adsorption, otherwise the loaded metals wouldn't be eluted from the carbon surface.
The adsorption of gold on carbon has been studied and reported in a survey of gold adsorption on carbon (Lodeishchikov and Panchenko in Using Activated Carbon for the Sorption of Gold from Acid Solutions of Thiourea, Tsvetn. Met., Vol. 9, No. 4, pp. 33-35, 1968). In this publication, test results on the loading capacity of charcoal using gold-containing acidified thiourea solutions were presented. Several different leach solutions were tested along with a solution obtained during the elution of gold from an ion exchange resin. The effects of pH, different ions in solution, and thiourea concentration were not studied, and the adsorption rate and adsorption isotherms were not determined. A separate test demonstrated that silver adsorption on activated charcoal was also possible.
In general, there has been very little work done on the elution of gold and silver from activated carbon where the precious metals are loaded from thiourea solutions.
Even in the process of cyanidation of gold and silver, the elution step has serious deficiencies, primarily with respect to kinetics. The elution of gold and silver from a cyanide solution can last from 10 hours to 4 days. The reason for the slow elution rate of the cyanide metal complexes is the strong adsorption of gold and silver cyanide complexes on activated carbon.
Furthermore, a complex desorption process was reported in the literature wherein thiosulfate ion was employed for desorbing silver from carbon loaded from a cyanide solution. More specifically, Veronese and Davidson in Chemical Abstracts 93:76833q disclose two desorbing steps to desorb gold and silver from activated carbon loaded from a cyanide solution. The first desorption step employs alkaline thiosulfate solution, and the silver is preferentially desorbed by three alkaline thiosulfate elutions leaving essentially all of the gold on the carbon. The gold is then desorbed by a second desorbing solution comprised of alkaline sodium cyanide. This abstract is based on South African patent No. 78/05464.
Other eluants are disclosed in the prior art for desorbing metals from carbon. For example, Heinen et al. in U.S. Pat. No. 4,208,378 disclose desorption of gold from activated carbon using a solution comprised of water soluble alcohol and aqueous base. Davidson et al. in U.S. Pat. No. 4,267,069 disclose regeneration of activated carbon using a carbonate or bicarbonate solution. Ross in U.S. Pat. 3,920,403 discloses a method of desorbing gold from activated carbon using a stripping liquid comprised of water, dilute caustic, or dilute caustic cyanide. Fischer in U.S. Pat. No. 3,935,006 uses ketones or alcohols to desorb gold from activated carbon. Davidson in U.S. Pat. No. 3,970,737 uses softened water to desorb gold from activated carbon loaded from a cyanide solution. Ogasa et al. in U.S. Pat. No. 4,595,572 disclose recovering gold and silver from an aqueous cyanic solution on activated carbon.